Hi
I'm new to the forum and am so far only on episode 550 so please go easy on me.
My theory about what may happen in the end simplified is as follows:
Ran accidentally gets involved with the Black organisation.
Conan has to rescue her.
Ran finds out about Conan and the formula, off Gin or someone in the organisation.
Ran and Conan along with Fbi defeat the Black Org boss etc.
Ran says to Conan she wants to be with him and takes the poison. Turns into little girl.
Ai says she will work on the cure, Ran replies don't worry take your time we are happy and she can spend longer with Shinichi.
The end credits happen.
After credits we see ran and Shinichi as adults again. Thinking the cure worked, then the adult detective boys burst in as well as Ai and the audience can now see this is in the future.
I believe Conan will never be cured as this will harm detective boys and be awkward.
I do think the easiest option would be Ran shrinking. it may also see Mouri and Eri get back together, reminding them of raising their daughter.